Chapter19.ppt

Based on McMurry, Organic Chemistry, Chapter 19, 6th edition, (c) 2003

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Chapter 19. Aldehydes and Ketones: Nucleophilic Addition Reactions

Based on McMurry’s Organic Chemistry, 6th edition©2003 Ronald KlugerDepartment of ChemistryUniversity of Toronto


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Aldehydes and Ketones

Aldehydes and ketones are characterized by the the carbonyl functional group (C=O)

The compounds occur widely in nature as intermediates in metabolism and biosynthesis

They are also common as chemicals, as solvents, monomers, adhesives, agrichemicals and pharmaceuticals


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19.1 Naming Aldehydes and Ketones

Aldehydes are named by replacing the terminal -e of the corresponding alkane name with –al

The parent chain must contain the CHO group The CHO carbon is numbered as C1

If the CHO group is attached to a ring, use the suffix See Table 19.1 for common names


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Naming Ketones

Replace the terminal -e of the alkane name with –one Parent chain is the longest one that contains the

ketone group Numbering begins at the end nearer the carbonyl

carbon


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Ketones with Common Names

IUPAC retains well-used but unsystematic names for a few ketones


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Ketones and Aldehydes as Substituents The R–C=O as a substituent is an acyl group is used

with the suffix -yl from the root of the carboxylic acid CH3CO: acetyl; CHO: formyl; C6H5CO: benzoyl

The prefix oxo- is used if other functional groups are present and the doubly bonded oxygen is labeled as a substituent on a parent chain


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19.2 Preparation of Aldehydes and Ketones

Preparing Aldehydes Oxidize primary alcohols using pyridinium

chlorochromate Reduce an ester with diisobutylaluminum

hydride (DIBAH)


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Preparing Ketones

Oxidize a 2° alcohol (see Section 17.8) Many reagents possible: choose for the specific

situation (scale, cost, and acid/base sensitivity)


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Ketones from Ozonolysis

Ozonolysis of alkenes yields ketones if one of the unsaturated carbon atoms is disubstituted (see Section 7.8)


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Aryl Ketones by Acylation

Friedel–Crafts acylation of an aromatic ring with an acid chloride in the presence of AlCl3 catalyst (see Section 16.4)


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Methyl Ketones by Hydrating Alkynes

Hydration of terminal alkynes in the presence of Hg2+ (catalyst: Section 8.5)


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19.3 Oxidation of Aldehydes and Ketones CrO3 in aqueous acid oxidizes aldehydes to

carboxylic acids efficiently Silver oxide, Ag2O, in aqueous ammonia (Tollens’

reagent) oxidizes aldehydes (no acid)


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Hydration of Aldehydes

Aldehyde oxidations occur through 1,1-diols (“hydrates”)

Reversible addition of water to the carbonyl group Aldehyde hydrate is oxidized to a carboxylic acid by

usual reagents for alcohols


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Ketones Oxidize with Difficulty

Undergo slow cleavage with hot, alkaline KMnO4

C–C bond next to C=O is broken to give carboxylic acids

Reaction is practical for cleaving symmetrical ketones


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Nucleophiles

Nucleophiles can be negatively charged ( : Nu) or neutral ( : Nu) at the reaction site

The overall charge on the nucleophilic species is not considered


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Electrophilicity of Aldehydes and Ketones Aldehyde C=O is more polarized than ketone C=O As in carbocations, more alkyl groups stabilize +

character Ketone has more alkyl groups, stabilizing the C=O

carbon inductively


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Reactivity of Aromatic Aldehydes

Less reactive in nucleophilic addition reactions than aliphatic aldehydes

Electron-donating resonance effect of aromatic ring makes C=O less reactive electrophilic than the carbonyl group of an aliphatic aldehyde


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19.6 Nucleophilic Addition of H2O: Hydration Aldehydes and ketones react with water to yield 1,1-

diols (geminal (gem) diols) Hyrdation is reversible: a gem diol can eliminate

water


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Base-Catalyzed Addition of Water

Addition of water is catalyzed by both acid and base

The base-catalyzed hydration nucleophile is the hydroxide ion, which is a much stronger nucleophile than water


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Acid-Catalyzed Addition of Water

Protonation of C=O makes it more electrophilic


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19.9 Nucleophilic Addition of Amines: Imine and Enamine Formation

RNH2 adds to C=O to form imines, R2C=NR (after loss of HOH)

R2NH yields enamines, R2NCR=CR2 (after loss of HOH)

(ene + amine = unsaturated amine)


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Mechanism of Formation of Imines

Primary amine adds to C=O Proton is lost from N and adds to O to yield a neutral

amino alcohol (carbinolamine) Protonation of OH converts into water as the leaving

group Result is iminium ion, which loses proton Acid is required for loss of OH – too much acid blocks

RNH2

Note that overall reaction is substitution of RN for O


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Imine Derivatives

Addition of amines with an atom containing a lone pair of electrons on the adjacent atom occurs very readily, giving useful, stable imines

For example, hydroxylamine forms oximes and 2,4-dinitrophenylhydrazine readily forms 2,4-dinitrophenylhydrazones These are usually solids and help in characterizing

liquid ketones or aldehydes by melting points


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Enamine Formation

After addition of R2NH, proton is lost from adjacent carbon

CC

O

C

C

O

H++ R2NH

NH

R

C

C

HON

R

C

C

H2ON

R

C

CN

R

+ H3O+

R

H

H HH

H H

R R R

H H H


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Protons on Carbons Adjacent to C=O

Slightly deshielded and normally absorb near 2.0 to 2.3

Methyl ketones always show a sharp three-proton singlet near 2.1


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Summary Aldehydes are from oxidative cleavage of alkenes, oxidation of 1°

alcohols, or partial reduction of esters Ketones are from oxidative cleavage of alkenes, oxidation of 2°

alcohols, or by addition of diorganocopper reagents to acid chlorides. Aldehydes and ketones are reduced to yield 1° and 2° alcohols ,

respectively Grignard reagents also gives alcohols Addition of HCN yields cyanohydrins 1° amines add to form imines, and 2° amines yield enamines Reaction of an aldehyde or ketone with hydrazine and base yields an

alkane Alcohols add to yield acetals Phosphoranes add to aldehydes and ketones to give alkenes (the

Wittig reaction) -Unsaturated aldehydes and ketones are subject to conjugate

addition (1,4 addition)

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Chapter19.ppt